With the gradual depletion of subterranean and shallow subsea hydrocarbon reservoirs, the search for additional petroleum reserves is being extended to deeper and deeper waters on the outer continental shelves of the world. As such deeper reservoirs are discovered, increasingly complex and sophisticated production systems have been developed. It is projected that by the year 1990, offshore exploration and production facilities will be required for probing depths of 6,000 feet or more. Since bottom founded structures are generally limited to water depths of no more than about 1,500 feet by current technology and because of the shear size of the structure required, other, so called compliant structures have been developed.
One type of compliant structure receiving considerable attention is a tension leg platform (TLP). A TLP comprises a semisubmersible-type floating platform anchored by piled foundations through vertically oriented members or mooring lines called tension legs. The tension legs are maintained in tension at all times by insuring that the buoyancy of the TLP exceeds its operating weight under all environmental conditions. The TLP is compliantly restrained in the lateral directions allowing sway, surge and yaw while vertical plane movement of heave, pitch and roll are stiffly restrained by the tension legs.
Several aspects of the design of the compliant structure concept are developed from dynamic considerations of the structure due to excitation by water waves. To minimize sway motions, the natural sway period of the structure must be either less than or greater than the wave periods at the various sea states. A stiff structure such as a fixed platform is designed with a natural sway period which is less than the wave period. However, the natural sway period of fixed platforms increases with increasing water depths and ultimately approaches the wave period resulting in large platform motions. In a compliant structure such as a TLP, the natural sway period is designed to be greater than the wave period.
Current TLP designs utilize heavy walled steel tubulars for the mooring elements. These tension legs constitute a significant weight with respect to the floating platform, a weight which must be overcome by the buoyancy of the floating structure. For instance, the tension legs utilized on the first commercial TLP installed in the Hutton Field of the British North Sea in 485 feet of water comprise steel tubulars having an outer diameter of 10.5 inches and an inner bore diameter of 3.0 inches. It should be readily apparent that, with increasingly long mooring elements being required for a tension leg platform in deeper and deeper waters, a floating structure having the necessary buoyancy to overcome the extreme weight of such mooring elements must be so large as to be uneconomic. Further, the handling equipment for installing and retrieving the long, heavy tension legs adds excessive weight and complexity to a tension leg platform system. Floatation systems can be utilized but their reliability is questionable. In addition, they cause an increase in the hydrodynamic forces on the structure.
In an effort to lower the weight of deep water tension legs while retaining the strength of the heavy steel tubulars, it has been proposed that high modulus composite structures 1 of carbon fiber and/or aramid fiber be employed While there is a significant reduction in the weight of such composite tension legs, composite structures are susceptible to impact damage. Furthermore, the relatively high cost of the raw materials renders the use of composites expensive and, thus, uneconomic for any installation other than to produce a large subsea oil bearing structure or in very deep waters.